Potato, Onion, and Mushroom Piroshki

1.In a measuring cup add buttermilk and 1/2 cup of vegetable oil, place in the microwave for 1 minute. Mix well.
2.In a bowl of a stand mixer add flour, sugar, 1 teaspoon salt, and yeast. Fit the mixer with the dough attachment and start on low. Gradually add in the buttermilk mixture. Wait until the dough comes together, add more flour if the dough is too sticky. Place the dough in a warm place covered with plastic wrap, and let rise for 2 hours until double in size.
3.To a small pot, add potatoes and 1 teaspoon salt then bring to boil. Cook on medium heat until fork tender.
4.To a skillet, on medium heat, add 1 tablespoon olive oil then onions, and fry until golden brown. Add mushrooms and dash of salt, garlic powder, thyme and ground pepper and fry until browned.
5.Once the potatoes are done add them to the onions and mushrooms then season with 1/4 teaspoon salt and black pepper to taste. Mix well. Add the truffle oil and parsley then mix again. Place mixture in the fridge until the dough is ready.
6.Line a baking sheet with parchment paper. Take the filling out and add mozzarella and parmesan cheeses then mix well.
7.Divide the dough into 16 even pieces. Flatten the dough into a disk with a small rolling pin and fill with potato mixture (about 2 tablespoons). Bring the ends up to the middle and pinch it close to make a small round bun. Lay on a baking sheet seem side down. Repeat with the rest of the dough. Cover and let it proof for 30 minutes.
8.Preheat oven to 350 F degrees. Get a small bowl for the egg and beat until combined. Brush each bun with beaten egg. Place in the oven and bake until golden brown on top, about 35 minutes. Transfer to a wire rack to cool, serve slightly warm.